In CSS, percentage (%) is a relative unit used to define the size, spacing, and positioning of elements based on the dimensions of their parent element. When a property like width, height, margin, or padding is set using a percentage, the value is calculated relative to the size of the parent container. For example, setting an element's width to 50% means the element will occupy half of its parent’s width. Percentages are widely used to create flexible and responsive web designs, allowing elements to adjust smoothly across different screen sizes. It’s important to note that for some properties, such as vertical padding and margins, the percentage is often calculated based on the width, not the height, of the containing block.
Pixels (px) are a fixed unit of measurement used in web design. They are based on the resolution of the display, meaning their size remains consistent across different devices and screen sizes. For example, an element set to 100px wide will always be 100 pixels wide, whether it’s viewed on a mobile phone or a large monitor. This makes px a great choice for elements that require precise dimensions, such as images, icons, or fixed layouts. However, since it doesn’t adapt to changes in the user’s browser settings or screen resolution, using pixels can lead to a lack of responsiveness and accessibility in some designs.